Milk thistle (Silybum marianum) is a flowering plant belonging to the Asteraceae (daisy) family. Milk thistle has a distinctive purple flower and spiky leaves, grows to about 200 cm in height, and is found throughout the world. Milk thistle is perhaps best known for its seeds, which have been shown to have liver cleansing effects. These hepatoprotective effects are through to relate to the presence of a group of flavonolignans called silymarin that are concentrated in the seeds of the plant. Silymarin is composed of the flavonolignans silybin A, silybin B, isosilybin A, isosilybin B, silychristin, isosilychristin, silydianin, as well as the flavonoid taxifolin. It is believed these compounds have significant antioxidant properties. By providing reducing power to the liver, they are able to prevent damage to cells and tissues caused by free radicals. However, milk thistle has other properties that may make it particularly effective at treating mood disorders. In this respect milk thistle may have anxiolytic properties.
Methamphetamine is able to cause significant changes to neurochemistry. In particular, methamphetamine is known to cause memory impairment and decrease serotonin in the hippocampus and dopamine levels in the prefrontal cortex. Silibinin is a mixture of silybin A and silybin B. Silibinin is able to attenuate the detrimental changes to neurochemistry caused by methamphetamine and these improvements correlate with the improvements in memory seen with milk thistle extracts.
For example, in one study, researchers investigated the anxiolytic properties of milk thistle in humans subjects with obsessive compulsive disorder (OCD). Subjects took either 30 mg per day of fluoxetine (a selective serotonin reuptake inhibitor), 600 mg per day of milk thistle extract or a placebo for 8 weeks. Both the fluoxetine and the milk thistle extract were effective at reducing the rating of obsessive compulsive disorder as measured by the Yale–Brown Scale, with no significant difference seen between the effectiveness of the two treatments. The effectiveness of milk thistle to reduce anxiety is possibly due to the ability of components of the herb to alter brain chemistry. For example, in mice extracts of milk thistle are effective at decreasing serotonin in the cortex, and increasing dopamine and norepinephrine levels in the cerebellum. These results suggest that milk thistle is effective at altering dopaminergic, serotonergic and noradrenergic activity. This may explain the mood elevating effects of the herb.
One way that milk thistle may affect neurochemistry is through modulation of monoamine oxidase enzymes. For example, it has been shown that the flavonoid taxifolin, a component of milk thistle seeds is able to inhibit monoamine oxidase B. This may explain the ability of the herb to increase monoamine levels in certain parts of the brain.
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